I've been going round and round with this, and I believe my answer is that ACS External Accounts does not have the capabilty that FileZilla has to Fetch a file.
I've been trying to set up the External Account to work like FileZilla (using SSH Key authentication) - however, I believe the issue is that I need to load the Private Key in the set-up and only the Public Key is allowed.
Can someone confirm that External Accounts does not have the capability for me set it up like FileZilla? Meaning, I generate the SSH Keys, load the Private Key in External Accounts, and then provide the client a Public Key? When I try accessing the FTP via a Transfer File activity, I get an 'access denied' error message. My assumption is because the Public Key is only loaded in ACS External Account (Support set this up for me).
So it all depends on what you're looking to do. If you need Campaign to transfer files to an external directory then you need only to upload the public key to Campaign as the private key should be present on the receiving server.
If you are trying to go in the reverse direction, you can submit a ticket to Campaign Support and our Ops team can provide you with a public key.
Typically you would never have both the private and public key loaded to the same location. If you are having access denied errors it could be a whitelisting problem.
I'd recommend reaching out to Campaign Support and we can investigate further.